The invention lies in the field of seismic geophysical prospecting. More particularly it relates to the processing of seismic signals. Still more particularly it relates to the processing of seismic signals derived from low energy sources, both impulsive sources of relatively short time duration, and low energy oscillatory sources of long time duration. Still more particularly, it relates to the digital processing of seismic signals from low energy sources.
In the early days of seismic exploration, the type of source used for initiation of seismic waves in the earth was an explosive material, such as dynamite. In reflection seismic operations, the dynamite charges were almost exclusively detonated in the bottom of a shallow bore hole, or shot hole, commonly of depth in the range of 50-100 feet, although at times as great as 500 feet or more. The shot hole was generally filled with water to tamp the charge, that is, to couple the explosive more tighlty to the earth.
The geophysicists early discovered that the surface layers of the earth were anomalous, in that they had a very low seismic propagation velocity. This led to errors in determining the travel time of the vertically travelling seismic wave through the surface layers, which was generally called the "weathered layer".
It was found that setting off a dynamite charge at the surface of the earth failed to give as much energy to a deep horizon, than if the same charge was detonated in a borehole at the base of the weathered layer. Therefore, in spite of the extra cost and time of drilling shot holes, and providing the water necessary to drill the holes, and for tamping the charges, this type of operation continued because of the need for information regarding the characteristics of the weathered layer.
This continued for many years until experiments were carried out to develop siesmic sources for use on the surface of the earth. The first of these was called the "weight drop". This involved the use of a large metal block that was lifted to a selected distance above the ground (about 8-12 feet), and suddenly released, permitting it to fall and impact the earth. This impulse on the ground surface did indeed send out a seismic wave which was reflected back from subsurface geological interfaces. However, the geophone signals recorded were extremely noisy, and by the customary process of visually examining seismic records, there was no evidence of the "reflections" that were easily perceived on records recorded from high energy charges in the shot holes.
It quickly becomes clear, that if any use was to be made of such low energy sources as the weight drop, there must be some way to add a large number of such noisy records so as to relatively increase the signal strength and reduce the noise. Saying this in another way, the signal to noise ratio of a single record from a low energy source is very low, too low to be used by conventional visual interpretational methods.
As a result of the need to add repetitive records, a magnetic recording system was devised, which was an analog recording system. Such systems continued in general use in the industry for recording records from low energy sources. To do this, the source and geophone positions were maintained the same and the signals from successive repetitions of the source were added in time synchronism. The successive records were recorded magnetically for the same position of the source and the geophones. The name given to the process was "adding", "stacking", "time stacking", and "compositing", etc. This was used with all types of weak sources, such as the weight drop, the "Vibroseis", and the "Dynoseis", and others, which subsequently came into general use.
The early stacking systems were analog magnetic recording and remained in general use for may years. Then, about 1965, there was a general change to digital magnetic recording of seismic signals. Such digital recording systems involved amplifiers of high gain, and variable gain, until today, the latest systems involve binary-gain-ranging amplifiers that can record digitally the amplitudes of seismic signals to 16 bits, or more.
Although the present low energy systems are applied to the surface of the earth, other means have been devised for overcoming the lack of precise velocity information in the weathered layer.
Also, about 15 years ago, there came into general use a different type of trace stacking or compositing. The stacking system previously described involved adding signals derived from the repetition of a source, where the two signals, or traces were added with their initiation times superimposed. That is, they were added in time alignment.
The new method of stacking was called "Common Depth Point" or "Common Reflection Point" stacking. These are generally referred to as C.D.P. and C.R.P. stacking. In adding traces in C.D.P., the traces must be from different sources and geophones. The important criterion is that all stacked traces must be reflected from the same depth point, or subsurface reflection point. All other portions of the travel paths of the traces are different.
While both time stacking and C.D.P. stacking improved the signal to noise ratio (S/N R) by partially cancelling random noise and adding signal, C.D.P. stacking had many other advantages not possessed by time stacking. Consequently, C.D.P. stacking came into wide use with conventional high energy sources, that is, large explosive charges, where high amplitude traces were recorded. Thus it became general practice to record seismic traces to 16 bits and then to C.D.P. stack.
In C.D.P. stacking, the "fold" of the stack, that is, the number of separate traces stacked to arrive at the final trace (such as two-fold, four-fold, twelve-fold, etc.) is very important. The larger the fold number, the better the S/N R.
C.D.P. stacking is not as simple to perform as is time stacking. In the latter simple synchronous adding of successive repetitions of a trace is sufficient. In C.D.P. stacking a great multiplicity of separate traces, each with different source point and geophone, must be stored digitally in a computer, and recalled in selected order. Considering that each trace is digitized at successive intervals of 0.001, 0.002, or 0.004 seconds, etc., with amplitudes recorded to 16 bits, great volumes of memory are required. For example, in Vibroseis operations, there may be 100-250 traces, or more, for each record, and each record may be recorded for 10-30 seconds, digitized at say 0.004 seconds to 16 bits. This adds up, conservatively to 20 million bits per record trace. So if 20-fold operations are to be carried out, more than 400 million bits must be stored.
Therefore, while high C.D.P. fold is desirable, because of the 16 bit signals and the large storage required, it has become common practice to time stack the traces (say up to 20 times) and then process by C.D.P. stacking, it being felt that the 16 bit digitizing is important, even in view of the poorer stack obtained.
Or, to put it another way, the time stacking of the weak signals was carried over from analog operations to digital processing. The C.D.P. stacking was carried over from high energy source work, where it was standard practice to digitize to 16 bits. So now it is standard paractice, with low energy sources to time stack to bring the signal amplitude up to where 16 bits is meaningful, and then to C.D.P. stack.
In the case of Vibroseis operations it has always been standard practice with digital processing to correlate the trace signals digitized to 16 bits with the sweep signal digitized to 16 bits.